Posts Tagged ‘phytophthora phaseoli’

The following was modified from a 2011 article from Bob Mulrooney, Extension Plant Pathologist (now retired), University of Delaware

As we move into late August and September, cooler temperatures, heavy dews and fogs, and the potential for heavy rains from tropical storms can be favorable for development of downy mildew in lima beans. Conditions are most favorable when fields receive 1.2 inches or more of rain within 7 days and when average daily temperature during this period is 78°F or less (heavy dews and fogs reduce the amount of rainfall necessary to start infection). Temperatures over 90°F will break the infection cycle. Lima bean fields should be scouted in the next 4 weeks for the presence of downy mildew as well as white mold. Race F of Phythophthora phaseoli was the only race of downy mildew identified from 2008 to 2011.

Preventative applications of 2 lbs fixed copper such as Kocide 3000 (1.3 lbs/A), Champ DP, or other coppers; 2 lbs Ridomil Gold/Copper; or 3- 4 pts Prophyt have provided control of downy mildew in the past. The best controls continue to be Ridomil/Gold Copper, and Prophyt, or other labeled phosphonate fungicides when disease pressure is high. Application at flowering or when pods are first forming is recommended if weather is favorable for disease. If disease is present Ridomil/Gold Copper and phosphonate fungicides have shown to provide some curative activity if applied when downy mildew is first seen. If downy is present in the field do not use copper fungicides alone for curative control, they will not provide control. Another product that is labeled on lima beans for white mold control is Omega and while not specifically labeled for downy mildew, three years of data has shown that it has excellent control of downy mildew at 5.5 fl oz and 8.0 fl oz/A as a preventative application before disease is found in the field. In DE this would be a 2ee use since the fungicide is labeled on lima beans and is appropriate where a mix of white mold and downy mildew are in the field. Omega has a 30 day preharvest interval. Headline from BASF is also labeled for downy mildew. It has been tested in Delaware and it has provided good control of downy when applied on a 10 day schedule at 6.0 fl oz. /A. It does not give as good disease control as Ridomil Gold/Copper or the phosphonates preventatively but the yields have been comparable. It is also labeled for anthracnose which the other products do not control. See the 2012 DE Commercial Vegetable Production Recommendations for more information on fungicides for lima beans.

We would appreciate samples and reports of lima bean downy mildew this season. Samples should be fresh and packaged in a zip lock plastic bag with dry paper towels. Samples can be dropped off at any of the county Extension offices or delivered directly to Nancy Gregory at Townsend Hall in Newark. Reports should be made to Nancy Gregory at ngregory@udel.edu.

Lima Bean Downy MildewDowny mildew is present now in Delaware lima bean fields. Levels are not high but many have been sprayed to protect the crop. Keep scouting and apply fungicides when needed; see past issues and the 2011 Delaware Commercial Production Recommendations for more information.

Fall Sanitation
In vegetable production it is not a good idea to leave old crop residue in the field any longer than necessary. If the crop is allowed to survive after harvest, fungi that cause many diseases continue to increase on the surviving plants. This allows higher numbers of the fungus to potentially survive until next season. Sanitation (plowing or disking the old crop) will help prevent pathogen carry-over.

Nematodes in Veggies
Fall is the best time to soil sample for nematode pests such as root knot, lesion, and other plant parasitic nematodes. After fall harvest but before any fall tillage is done take soil cores six inches deep between plants in the row. Samples should be taken in the root zone of the old crop. Twenty cores/ sample should be taken from random spots in the field and placed in a plastic bucket gently mixed, and a pint of soil submitted for analysis. Large fields should be subdivided into blocks of 10-15 acres each and sampled separately. Nematodes are not uniformly distributed in the soil and it would be easy to miss significant numbers if a single sample of 20- 25 soil cores represented a large acreage. Nematode test bags and instructions are available for purchase from the county Extension offices. Samples cost $10.00. Fall sampling for root knot nematodes is strongly recommended for fields that will be planted in cucumbers, watermelons, cantaloupes, lima beans or other high value vegetables where root knot could reduce production. Forms and instructions are also available on the web at http://ag.udel.edu/extension/pdc/index.htm.

New video on nematode sampling
“How to Sample for Nematodes” is a new video that was just produced to help growers with taking nematode samples in the fall to monitor plant parasitic nematode populations in their fields. The video features Bob Mulrooney, Extension Plant Pathologist at the University of Delaware explaining and demonstrating how to take soil sample for nematodes in row crops as well as narrow crop soybeans. The link for viewing is on the CANR You Tube server at http://youtu.be/x5HcY_L6aQk.

Lima bean downy mildew was found by a CCA and confirmed on Wednesday from a field of ‘C-elite’ near Galena, MD. Growers need to be scouting carefully and applying fungicides as needed. If seen in the field apply either Ridomil Gold/Copper 2.0 lbs/A or ProPhyt (3.0 pts/A). See the 2011 Delaware Commercial Vegetable Production Recommendations for other fungicide choices as well as last week’s WCU for more detailed information.

Basil downy mildew was found in New Castle County this week. Any specialty crops growers might want to protect basil with one of the phosphorus acid products, such a ProPhyt, at this time.

Powdery and downy mildew are widespread in cucurbits especially pumpkins and winter squash at this time. Maintain fungicide programs until fruit develop fully.

Unfortunately Phytophthora fruit rot is very prevalent on a number of cucurbits especially pumpkin at this time. The excessive rainfall just made a bad problem worse. A few growers have asked about dipping fruit in a 5-10% bleach solution or using Zerotol to prevent fruit rot. My experience has been that is not effective if the fruit are infected in the field. You may get reduced spread in a bin but it will not control Phytophthora fruit rot.

There were a few reports of late blight in New York and Connecticut this week, but nothing in the Mid-Atlantic to worry tomato growers so far. To track the progress of late blight in the US you can go to http://usablight.org

The recent heavy rainfall from the storms has been very favorable for development of downy mildew. Lima bean fields should be scouted carefully now for the presence of downy mildew as well as white mold. The weather has also raised the possibility of seeing gray mold (Botrytis). Race F of Phythophthora phaseoli was the only race of downy mildew identified in 2006, 2008, 2009, and 2010. Preventative applications of 2 lbs. fixed copper, 2 lbs. Ridomil Gold/Copper, or 3- 4 pts. Prophyte have provided control of downy mildew in the past. The newest formulation of fixed copper from DuPont is Kocide 3000 and it performs as well as the other formulations of copper at the rate of 1.3 lbs/A. The best controls continue to be Ridomil/Gold Copper, Prophyt, or other labeled phosphonate fungicides and Omega, especially when disease pressure is high. Application at flowering or when pods are first forming is recommended if weather is favorable for disease. If disease is present Ridomil/Gold Copper and phosphonate fungicides have shown to provide some curative activity if applied when downy mildew is first seen. If downy is present in the field do not use copper fungicides alone for curative control, they will not provide control. Another product that is labeled on lima beans for white mold control is Omega but not downy mildew, but in DE this would be a 2ee use that someone like me can recommend since the fungicide is labeled on lima beans. I have three years data that show excellent control of downy mildew at 5.5 fl oz and 8.0 fl oz/A as a preventative application (before disease is found in the field). Omega is not labeled for aerial application, however. Headline from BASF is also labeled for downy mildew. I have tested it and it has provided good control of downy when applied on a 10 day schedule at 6.0 fl oz. /A. It does not give as good disease control as Ridomil Gold/Copper or the phosphonates preventatively but the yields have been comparable. It is also labeled for anthracnose which the other products do not control. See the 2011 DE Commercial Vegetable Production Recommendations for more information on fungicides for lima beans.

Downy mildew caused by Phytophthora phaseoli

Downy mildew on raceme and petiole

Phytophthora capsici on lima bean pod.

Phytophthora capsici will infect lima bean pods as well and can look very similar. P. capsici or lima bean pod rot is usually found in wet low spots in the field. The fungus growth looks more granulated or “pebbly” than downy mildew, microscopic confirmation is encouraged.

Downy mildew on the upper pod and lima bean pod rot on the lower pod. Note the granular appearance of the fungus on the lower pod and the lack of a reddish brown border on the pod infected with lima bean pod rot or Phytophthora capsici.

Cucurbit Downy MildewCucurbit downy mildew was confirmed on pumpkin, summer squash and winter squash this week in DE. It will likely be more widespread following the storm. Maintain fungicide sprays specific for downy mildew if the foliage is still green and harvest is still planned.

Late Blight
Late blight was reported in a central New York county this past week on tomatoes in a homeowners garden. After the hurricane we might see some late blight develop on tomatoes.

With the impending hurricane we will probably see large amounts of rainfall. For vine crops fields with a history of Phytophthora fruit rot it will mean more Phytophthora fruit rot if any marketable watermelon or cantaloupe remain but it will really threaten the pumpkin crop. No fungicide will protect fruit from fruit rot if we get huge amounts of rainfall. Standing water in the fields will be the biggest indicator of possible fruit rot damage. There would be some benefit to protecting foliage with fungicides before the storm arrives if there is time and there is no history of Phytophthora fruit rot. This would be true for many vegetables including vine crops, tomatoes, and others. If there is no Phytophthora fruit rot present in a pumpkin field, fungicides such as Presidio, Ranman, Revus or Forum plus a fixed copper could be considered to suppress Phytophthora fruit rot, if Phytophthora spores moving in water from other fields should be introduced. Fruit have to be covered for the materials to have a hope of having an effect.

For other vegetables such as tomatoes, peppers and other fruiting vegetables that may be planted for late harvest fungicide applications should be made before the rains arrive not after. Prevention is the key to control. If the label allows, adjuvants that help products adhere to the plants should be considered. Spreader-stickers would be encouraged if the crop and label warrant it.

In crops where cottony leak caused by Pythium could cause crop loss, such as snapbeans and lima beans, application of one of the phosphonate fungicides such as ProPhyt or Phostrol would be suggested at maximum rates, or Ridomil Gold/Copper on snapbeans only. There is a 24c label for Ridomil Gold/Copper (2.0 lbs/A) in DE, MD and VA for cottony leak on snapbeans. Lima bean growers will want to scout carefully once this storm clears out for downy mildew. Wet soil and cool temperatures will favor downy mildew infection.

Cucurbit downy mildew is present on pumpkin now in the sentinel plot in Newark in New Castle County. This is the first report of downy mildew on pumpkin. It has probably been there for several days. Growers should continue to apply fungicides for leaf diseases including downy mildew.

Sweet Corn
With a return to some cooler weather be on the lookout for corn leaf rust on sweet corn. Scout the plantings and if you see rust on plants at the whorl stage or younger, rust could become an issue if the hybrid is not resistant to rust. Rust, when heavy, can affect plant health and reduce ear size. The best control is to plant resistant hybrids, but the strobilurin or triazole fungicide work well. On fresh market corn rust on the husks makes ears unsightly to consumers.

Cucurbits
Cucurbit downy mildew was recently observed on cantaloupe in the sentinel plot in Newark. These lesions resemble the same symptoms as seen on cucumber but spore production appears to be very sparse. It has not moved to any pumpkin, winter squash or watermelon so far. The susceptible cucumber in the plot is almost totally defoliated.

Lima Beans
So far weather conditions have not been favorable for downy mildew. It looks like the weather may be changing and getting a bit cooler with more dew and possibly fog in the early morning hours. If it should start raining soon growers and crop consultants should be scouting for downy mildew. Race F of Phythophthora phaseoli was the only race identified in 2006, 2008, 2009, and 2010. Preventative applications of 2 lbs fixed copper, 2 lbs Ridomil Gold/Copper, or 3- 4 pts Prophyte have provided control of downy mildew in the past. The newest formulation of fixed copper from DuPont is Kocide 3000 and it performs as well as the other formulations of copper at the rate of 1.3 lbs/A. The best controls continue to be Ridomil/Gold Copper, Prophyte, or other labeled phosphonate fungicides, and Omega, especially when disease pressure is high. Application at flowering or when pods are first forming is recommended if weather is favorable for disease. If disease is present Ridomil/Gold Copper and phosphonate fungicides have shown to provide some curative activity if applied when downy mildew is first seen. If downy is present in the field do not use copper fungicides alone for curative control, they will not provide control. Another product that is labeled on lima beans for white mold control is Omega but not downy mildew, but in DE this would be a 2ee use that someone like me can recommend since the fungicide is labeled on lima beans. I have three years data that show excellent control of downy mildew at 5.5 fl oz and 8.0 fl oz/A as a preventative application (before disease is found in the field). Omega is not labeled for aerial application, however. Headline from BASF is also labeled for downy mildew. I have tested it and it has provided good control of downy when applied on a 10-day schedule at 6.0 fl oz /A. It does not give as good disease control as Ridomil Gold/Copper or the phosphonates preventatively but the yields have been comparable. It is also labeled for anthracnose which the other products do not control.

Downy mildew caused by Phytophthora phaseoli

Downy mildew on raceme and petiole

Phytophthora capsici on lima bean pod.

Phytophthora capsici will infect lima bean pods as well and can look very similar to downy mildew. P. capsici or lima bean pod rot is usually found in wet low spots in the field. The fungus growth looks more granulated or “pebbly” than downy mildew, microscopic confirmation is encouraged.

Downy mildew on the upper pod and lima bean pod rot on the lower pod. Note the granular appearance of the fungus on the lower pod and the lack of a reddish brown border on the pod infected with lima bean pod rot or Phytophthora capsici.

Nematodes in Veggies
Fall is the best time to soil sample for nematode pests such as root knot, lesion, and other plant parasitic nematodes. After fall harvest but before any fall tillage is done, take soil cores six inches deep between plants in the row. Samples should be taken in the root zone of the old crop. Twenty cores/ sample should be taken from random spots in the field and placed in a plastic bucket, gently mixed, and a pint of soil submitted for analysis. Large fields should be subdivided into blocks of 15- 20 acres each and sampled separately. Nematode test bags and instructions are available for purchase from the county Extension offices. Samples cost $10.00. Fall sampling for root knot nematodes is strongly recommended for fields that will be planted in cucumbers, watermelons, cantaloupes, lima beans or other high value vegetables where root knot could reduce production. Forms and instructions are also available on the web at http://ag.udel.edu/extension/pdc/index.htm. Just a reminder, as I mentioned last week, do not take nematode samples until we get some rainfall if the soil is very dry.

Fall sanitationIn vegetable production it is not a good idea to leave old crop residue in the field any longer than necessary. If the crop is allowed to survive after harvest, fungi that cause many diseases continue to increase on the surviving plants. This allows higher numbers of the fungi to potentially survive until next season. Sanitation (plowing or disking the old crop) will help prevent pathogen carry-over.

Lima Beans
To date, I have not heard about or seen any samples of downy mildew in lima beans. The weather has not been favorable for infection.

I was disappointed to learn that our 24c special local needs label for Phostrol fungicide for downy mildew has expired. This means that it is not registered for use on lima beans in Delaware. NuFarm Americas is pursuing labeling of Phostrol for legume vegetables, including limas, so by next season we may not need the 24c label to use this product. The active ingredients in Phostrol are mono- and di-potassium salts of phosphorous acid, this is one of several fungicides that we call phosphonate fungicides. These same ingredients are found in Prophyte and K-phite which have labels for use on beans for downy mildew. I have tested Phostrol and a product that was never marketed here called Fungi-phite — both are excellent for downy mildew control. I would feel very safe to say the other labeled products with these same active ingredients should perform as well. I have Prophyte in my trial this year so I will be able to say for sure, if the test is successful.

In last week’s article I went over the control measures for downy mildew on baby lima beans and recommended Omega as a new product labeled for lima beans that has excellent activity for downy mildew as well as white mold. The label is a little ambiguous about recommended application methods and it was clarified by the manufacturer that it is to be applied by ground only, not by air. I am in contact with the company concerning this issue, and they will be pursuing changing the label with EPA. Unfortunately nothing will happen in time for this season if downy mildew should appear later, but it can be applied by ground rigs now.